For many years the main approach to measuring flow in a gas or air stream was with a device referred to as a pitot. Typically the pitot was provided with two pressure openings--one for measuring the static pressure and one for measuring total pressure which included the combined static and velocity pressure. As is well appreciated and known in the industry, the single pitot approach has serious shortcomings. Among such shortcomings is the fact that the design only effectively indicates velocity at one point in the flow stream. In order to achieve more accurate readings with a single pitot tube, it has been necessary to scan the flow stream with the pitot and to average the readings taken at selected points. However, this approach is not withouts its difficulties. To even scan the flow stream requires a traversing drive mechanism for the pitot. In addition, the pitot must be maintained in a proper position with respect to the moving air or gas of the flow stream. Coupled to this is the problem of the pitots tendency to become clogged or plugged with foreign material passing within the stream of flow, especially in industrial type applications. Thusly it can be said that the single pitot has never been really effective enough to become widely accepted in industry and used in industrial installations. Of late, however, the industry has seen the emergence of averaging pitot systems that are specifically designed to sense and average both total and static pressure over the entire cross section of the duct housing the passing flow. These systems generally include multibranches disposed about the cross section of the duct and wherein the respective branches are provided with multi-pressure sensing points, either the port or pitot type, for averaging both static and total pressure of the fluid passing within the system. One is referred to the disclosures found in U.S. Pat. Nos. 3,685,355; 4,036,054; and 3,981,193 for a more complete and unified understanding of basic approaches to pressure averaging primaries.
Generally, the pressure averaging primaries referred to above are designed and fabricated within a containing duct which is mounted or inserted within an existing duct at the site of installation.
In addition the pressure averaging primaries of the prior art has not been totally suited from a design standpoint, for very large duct applications. Heretobefore, situations with very large duct designs have required that the pressure averaging primary be completely custom designed and manufactured. Obviously this is expensive and requires a substantial amount of design and manufacturing time.
Therefore, there has been and continues to be a need for a pressure averaging primary of a basic production type design that is relatively simple and inexpensive and which can be easily and conveniently retrofitted to an existing duct.